1firstPOSTCallToAPI('url', data).pipe(
2    concatMap(result1 => secondPOSTCallToAPI('url', result1))
3      concatMap( result2 => thirdPOSTCallToAPI('url', result2))
4       concatMap(result3 => fourthPOSTCallToAPI('url', result3))
5    ....
6).subscribe(
7    success => { /* display success msg */ },
8    errorData => { /* display error msg */ }
9);
10